English Language Center @ UVIC
Click on the first 5 links in the section called Verbs.  You will find theory as well as exercises to practice.
 

ESL Blues
Scroll down the page to find all the exercises on the present progressive, the present simple, and the present simple vs the present progressive.
 

English Zone
Specific information on frequency adverbs.
 

More from English Zone
Do you know your non-action verbs well?  Check the chart and try the quiz.